The present invention relates generally to rate sensing devices and systems, and more particularly to a novel electrofluidic angular rate sensing system, and associated apparatus and methods.
Various attempts have previously been made to design and build a fluidic replacement for the mechanical rate gyroscope long used as the primary attitude sensing element of conventional navigational guidance systems for ships, planes, guided missiles and the like. The most common approach has been to employ a device known as the fluidic angular rate sensor.
Such device basically comprises a body in which a chamber is formed. Pressurized air is forced through a nozzle passage within the body to form a jet which traverses the chamber. Spaced apart from the exit of the nozzle passage, and positioned directly in the path of the jet is a splitter designed to divide the jet into two separate and equal streams when the sensor body is at rest. As the sensor experiences rotation about a control axis (of the ship, plane, missile or the like) perpendicular to the axis of the nozzle passage, the splitter unequally divides the jet in a proportion representative of the rate and sense of such rotation. This unequal jet division during rotation about the control axis results from a relative offset between the splitter and jet caused by the Coriolis effect.
Each of the unequal streams flows into a different one of a pair of receiving passages positioned on opposite sides of the splitter within the body. The streams cause a pressure (or flow rate) differential between the receiving passages which is indicative of the rate and sense of the body's rotation about its control axis. Such pressure or flow differential may thus at least theoretically be used to generate and transmit corrective input signals to other components of the guidance system to thereby return the ship, plane, missile or the like to the correct attitude relative to the control axis.
Heretofore the fluidic replacement of the rate gyroscope, and navigational rate sensing systems utilizing it, has been hindered by a variety of structural and functional problems associated with conventional fluidic angular rate sensors. For example, unavoidable fabricational inaccuracies in such devices have prevented them from obtaining the extreme accuracy needed to replace the gyroscope. More specifically, despite the use of modern precision manufacturing techniques, certain internal asymmetries and misalignments remain which result in unequal division (or "offset") of the jet at zero rotational rate of the sensor about its control axis. This offset, of course, introduces a continuing source of output error into the operation of the sensor.
Greatly aggravating the jet offset problem is the environmental sensitivity of conventional fluidic rate sensors. Changes in the environment to which the conventional sensor is exposed cause its jet to variably drift relative to the splitter, thereby adding another source of unacceptable sensor output error.
Another equally vexing problem has been that of obtaining a useful (i.e., sufficiently powerful, accurate and responsive) output signal from the conventional fluidic rate sensor. It is desirable to convert the initial fluidic outputs of the sensor to electrical signal outputs in order to conveniently integrate the sensor with the electrical control surfaces of the guidance system (e.g., the autopilot system of an airplane). In addition to being unacceptably inaccurate because of the above-mentioned offset and drift problems, such initial fluidic outputs are quite weak. Thus, great difficulties have been encountered in using them to drive pressure-to-electric transducing devices to obain electrical output signals. Attempts to utilize hot wire anemometer circuitry, wherein sensing wires are placed in each of the sensor receiving passages to monitor the varying flow rate differentials therebetween, have proven equally unsuccessful due to unacceptably high response times involved in differentially cooling such sensing wires.